1. Field of the Invention
This invention relates to a differential amplifier, and more particularly to a differential amplifier having a reduced number of circuit elements, and providing an excellent common mode signal rejection ratio (CMRR), and a high operational stability.
2. Description of the Prior Art
Conventionally, such differential amplifiers as shown in FIG. 10 have been known as those having a small number of resistive elements and adapted for integration in a monolithic semiconductor integrated circuit. The details of such a differential amplifier is disclosed in IEEE International Solid State Circuits Conference Digest of Technical Papers, pages 16 to 17, published on Feb. 19, 1969. As shown in FIG. 10, the circuit employs a current source circuit consisting of diode D.sub.1 and a pnp-type transistor Q.sub.3 as the load for a pair of differential transistors Q.sub.1 and Q.sub.2. Explaining the operation of such a current source circuit with the circuit as shown in FIG. 11 in which a diode D is connected between the base and the emitter of a transistor Q, if the pn-junction of the diode D and the base-emitter junction of the transistor Q are applied with completely the same forward bias voltage V.sub.BE and the diode and the transistor are manufactured by simultaneous impurity diffusion, the V-I characteristics of their pn-junctions are equal to each other and they allow currents of the same current density to flow therethrough. Therefore, letting the base-emitter junction area of the transistor Q be A.sub. Q and the pn-junction area of the diode D be A.sub.D, the relation between the emitter current I.sub.Q of the transistor Q and the diode current I.sub.D of the diode D will follow the relation, EQU (I.sub.D /A.sub.D)=(I.sub.Q /A.sub.Q.
therefore, in such a type of circuit, when a current source circuit including a diode and a transistor having junction areas of equal size is used as the load for a pair of differential amplifying transistors Q.sub.1 and Q.sub.2, a differential signal current i.sub.1 is allowed to flow through the pair of differential transistors Q.sub.1 and Q.sub.2 in the direction of the arrow and the pn-junction of the diode D.sub.1 in said current source circuit is biased by this differential signal current i.sub.1 to generate a forward voltage V.sub.BE. The base-emitter of the pnp-type transistor Q.sub.3 is biased by this forward voltage V.sub.BE. Then, since the junction areas of the diode and the transistor are equal to each other, a signal current i.sub.2 having the equal current value with said differential signal current i.sub.1 is allowed to flow through said transistor Q.sub.3 in the direction of the arrow in the figure. Therefore, at the circuit interconnection P of the differential output connected with the collectors of the transistors Q.sub.2 and Q.sub.3, a differential output signal current of 2i(= i.sub.1 + i.sub.2) is obtained which is twice as large as the differential output signal of the usual differential amplifier and provides a high gain.
Further, the biasing currents of the differential pair transistors Q.sub.1 and Q.sub.2 of this differential amplifier are set at constant current values by the constant current source circuit consisting of a resistor R, a diode D.sub.2 and a transistor Q.sub.4. Therefore, said biasing currents are subjected to no change upon any common mode signals applied to the respective bases (pin 1 and pin 2) of the differential pair transistors Q.sub.1 and Q.sub.2, enabling the provision of an excellent common mode signal rejection ratio (CMRR). To the differential output circuit interconnection P of said differential amplifier, a current amplifying circuit of the next stage consisting of transistors Q.sub.5 and Q.sub.6 is connected to supply a signal current which is current-amplified signal of said differential output signal to an output terminal (pin 3).
The conventionally known differential amplifier provided with a next stage current amplifying circuit as described above, however, has such drawbacks that the number of circuit elements is large, i.e. six transistors, two diodes and one resistor (nine in total), and that the cost of the differential amplifier becomes high.